1. Field of the Invention
The present invention relates to telephone networks, and more particularly to a system and method for accessing, monitoring and testing a telephone network.
2. Description of the Related Technology
The telephone industry has changed drastically since the divestiture of the Bell System. Today, seven regional Bell Operating Companies (RBOCs) and independent telephone companies provide local telephone service within 166 Local Access Transport Areas (LATAs). These companies are forced to rely on interexchange carriers such as AT&T, MCI and Sprint for transmission of calls from one LATA to another. The responsibility for quality and performance of the telephone circuit is thus split between local telephone companies and interexchange carriers.
RBOCs are under pressure for financial performance as independent companies. With rates restricted by utility commissions, and facing rising costs and new competition with restrictions on transmission of calls beyond LATA boundaries, telephone companies find themselves faced with rapid introduction of new technology, smarter business customers, and antiquated network maintenance systems.
Demand for information transmission increased dramatically during the 1980s. At the beginning of the decade most data transmission systems interfaced a predominantly analog network through relatively low speed modems. Those desiring "high speed" transmission generally opted for the 56 kbps rate of the digital data system (DDS).
Considerable pressure for increases in such transmission rates came from the desire to take advantage of the increasing capabilities and computation speeds of computers and other business systems. Improvements in transmission technology during the decade fueled the momentum of the increase in transmission rates. The replacement of copper cables with glass fiber expanded the transmission capacity of outside plants many times over. At the same time, improvements in electronics and coding algorithms yielded terminal equipment designed to take advantage of the enormous increase in bandwidth which accompanied the conversion to fiber optics.
In the absence of a standard, virtually all lightwave vendors chose DS3 (44.736 Mbps) as the interface between the lightwave terminal and the network. FIG. 1 illustrates the prior art North American Digital Hierarchy having a DS0 (64 kbps) level 102, a DS1 (1.544 Mbps) level 104, a DS2 (6.312 Mbps) level 106 and a DS3 (44.736 Mbps) level 108. This hierarchy is defined by ANSI T1.102-1987--"Digital Hierarchy, Electrical Interfaces", The American National Standards Institute, Inc., New York, 1987. DS2 is important as a link between DS1 and DS3. Even though there is little growth in DS2 as a transport medium, the DS2 level exists in every muldem (multiplexer/demultiplexer) or other network element which must interface DS1 and DS3 signals. Although DS0 is essentially confined to digital signals, reference to analog voice frequency signals is included in FIG. 1 because of widespread interfacing of such signals to the DS1 level of hierarchy by digital channel banks.
The transition of telecommunications into the 1990s will thus occur with the DS3 rate used almost universally for interfaces within the network. DS1 transmission between customers and operating companies is now commonplace, and an ever increasing number of customers are seeking to interface with service providers and with other end users at even higher rates. The DS2 rate, seemingly a logical intermediate step between DS1 and DS3, has proved to be uneconomical for transport except in certain special cases. Thus, DS3 is proving to be the underlying building block for high bandwidth, light signals.
FIG. 2 is a prior art, simplified model of a lightwave network 120 showing four example network carriers (Carrier A, Carrier B, Carrier C and Carrier D) and how a DS0 level line 130, a DS1 level line 132, a DS3 level line 134 and a fiber optic (light) line 136 are used to interconnect a customer X 140 to a customer Y 142. The equipment at the customer premise or site 140 and 142 could be, for example, a telephone, a facsimile modem or a data modem.
A multiplexer/demultiplexer or channel bank 144 is used to multiplex 24 DS0 level signals on the line 130 into one DS1 level signal on the line 132. In this model 120, a M1/3 muldem 146 is used to multiplex 28 DS1 level signals on the line 132 into one DS3 level signal on the line 134. The DS3 level signals on the line 134 are further combined by Carrier A using a lightwave transport multiplexer 122 into a fiber optic signal on the line 136. In this model 120, three Central Offices 152, 154 and 156 are used, with the middle Central Office 154 having three carriers cross-connected at the DS3 level by use of a cross-connect 158.
A long distance call from customer X 140 to customer Y 142 involves many levels of multiplexing and many transport carrier handoffs. Carrier A is the local operating company of customer X 140, and owns Central Offices 152 and 154. Carrier B and Carrier C are long distance carriers, and Carrier D is the local operating company that owns Central Office 156 and services customer Y 142.
A call from customer X 140 to customer Y 142 involves three central offices and three transport carriers. As the call traverses the network 120, it may be processed by several network elements, such as channel banks 144, M1/3 muldems 146, 128, and lightwave transport multiplexers 122, 126 with each element having its own surveillance techniques. Maintenance and billing problems are not uncommon with this interaction.
Most network elements incorporate some form of monitoring, test, and control of the data that they process. However, none of these options supports the continuous monitor or test access of DS3 and all embedded channels.
Although the cost of bandwidth has plummeted to the extent that it no longer worries facility planners as it did in previous decades, the move to DS3 is not without its costs. Chief among them are the lack of convenient and economical test access to lower rate channels embedded in the DS3 bit stream and the lack of surveillance systems designed to take advantage of the performance data embedded in the DS3 formatted signal.
DS3 (and to a lesser extent DS1) signals carry large amounts of data per unit time and represent a considerable financial investment on the part of the end user, for whom bandwidth is not as inexpensive as it has become for the operating company facility planner. The operating company using DS3 runs the risk of a substantial outage in the case of a crippling impairment or total failure of such high-speed digital facilities. Those who manage the DS3 facilities of both end users and service providers are thus quite interested in the performance of the digital links in their networks. They are not satisfied to let the performance information embedded in the bit streams they deal with simply pass on by without extracting data which can be quite useful in managing the network and in minimizing the costly impact of service outages.
It is possible to acquire a DS3 signal at the monitor jack of a DSX-3 cross-connect panel and demultiplex from the DS3 whatever subsidiary signals are desired. Such signals may then be patched into portable test equipment or routed to test systems for analysis. There are many test sets available which will analyze signals extracted at any rate from DS0 to DS3. This technique, however, requires manual access to implement the patching and allows the use of the test and/or surveillance equipment on only one DS3 at a time. Portable test arrangements of this type do not generally allow the insertion of test signals or data into outgoing channels of a DS3 bit stream without interrupting the other services carried by the same DS3.
A digital cross-connect system (DCS) might be considered for use as a test access vehicle in the DS3 network. The versatile and sophisticated switching capabilities of the DCS make it a costly access. There are, in addition, impairments associated with the use of DCS which make it inadvisable to scatter such systems throughout the network at all points requiring surveillance or test access. Among the impairments introduced by a DCS are delay, a certain amount of which is necessary to synchronize incoming and outgoing frame structures, and robbed-bit writeover distortion, the latter difficulty occurring only when switching down to the DS0 rate is provided.
To improve service while cutting costs, RBOCs have turned from portable test equipment in the hands of field craftspeople, to permanently installed test systems connected to a central network management center or operations support system (OSS); and from repair actions in response to a trouble report from customers, to proactive network performance monitoring and preventative maintenance. Operations support system (OSS) is also known as operations system (OS). Existing equipment available to telephone companies provides only a small portion of the functionality needed by telephone companies and is quite expensive.
Thus, a base device that provides full test access and continuous performance monitoring functionality of a large number of DS3 channels and all embedded channels has been invented (see Related Applications). This device provides comprehensive, full-time performance monitoring of DS3 (High-Speed Subsystem) and embedded or directly connected DS1 (Low-Speed Subsystem) circuits, and testing of DS1, DS0 and subrate circuits, along with an extensive suite of test capabilities for HiCap, DDS and VF services.
What is now desired is a cost-effective access and test solution for small central offices or end-user installations that support fewer DS3 or DS1 circuits. Hence an architecture having one or more remote devices with similar functionality as the base device. With a distributed architecture, or system, the cost of the common equipment inside of the base device is spread over multiple remote sites. A distributed architecture system would provide remote test and performance monitoring to customer premises, end-offices and collocated Competitive Access Provider (CAP) sites, as well as more typical Interexchange Carrier (IEC) Point of Presence (POP) locations and cellular applications. The comprehensive performance monitoring and testing capabilities of a base device can be extended to the entire network by the distributed architecture, thus obsoleting previous generations of test equipment.
Such a distributed architecture system should provide a cost effective single point interface to Operations systems (OS) such as ITS, NMA and SARTS from the base device. This feature would eliminate multiple OS links to each of a plurality of remote devices of a distributed architecture system.
The use of a communications link between a main unit and some remote equipment, and the testing of the operation of the link and equipment, is described in U.S. Pat. No. 5,027,343 to Chan, et al. However, the Chan system does not have the capability to test the traffic at the remote location. U.S. Pat. No. 5,271,000 to Engbersen, et al., discloses testing of packet switched networks. However, the Engbersen system does not have the capability to test the traffic of circuit switched networks.
U.S. Pat. No. 5,018,136 to Gollub describes a system wherein received speech samples are digitized, transformed into packets and asynchronously retransmitted. U.S. Pat. No. 5,138,440 to Radice describes a system for communicating asynchronous video/television signals over a digital path by use of oversampling/overstuffing methods.
However, what is desired is to be able to test the traffic of circuit switched networks at a location remote from a base location without the use of oversampling or overstuffing to assure data integrity. What is also desired is a mechanism to align incoming and outgoing channels at a location remote from a base location such that the end to end path delay is eliminated and the communication channels, e.g., DS0, are not skewed.